1. Field of the Invention
The present invention generally relates to a digital speech transmission/reception system. More specifically, the present invention is related to a digital speech transmission/reception system, which is composed of a digital speech transmitting unit, and a digital speech receiving/reproducing unit, and in which an error data portion of a digital speech data transmitted via a radio signal transmission path can be replaced by a speech synthesized data.
2. Description of the Related Art
As a conventional speech transmission system, for instance, a mobile radio communication system using a speech compressing method such as the ADPCM method and the V-SELP method is known. In order to correct an error portion of a speech data produced due to the fading effect in such a communication system, the error portion of the speech data is replaced by a portion of the speech data positioned immediately before the error portion of the speech data. Also, in another error correcting method, the speech data is reproduced with a low sound volume while the error portion of the speech data is reproduced. Such a technique is described in, for example, Japanese Laid-open Patent Application (JP-A-Heisei 7-240711).
However, there is a case where digital speed transmission is performed under a deteriorated radio signal transmission path, namely under low electric field strength for a radio signal, or under large change in electric field strength such as mobile communications from a vehicle driven through buildings. In such a case, it is practically difficult to transmit a digital speech data in perfect condition without any loss of the digital speech data. Even in such worse condition, in a bidirectional communication system, a data re-send request is issued and the speech data is received again so that the complete digital data can be received in practically perfect condition.
To the contrary, in a unidirectional radio transmission system, since a data re-send request is not allowed, it is necessary to issue the date re-send request by utilizing a wire communication system such as a telephone line.
This may be effective in acquisition of correct information. However, in a speech transmission system, when the content of speech is necessary to be understandable, since the immediate speech transmission has a top priority, the above-described system could not be practically accepted.